Sinclair ZX80 / ZX81 / Z88 Forums

Would you say the design itself is wrong ,Mark or is it a case of this module is for another computer but being peddled as ZX81 compatible?

I would say that it was intended for the ZX81. But this is based on it being a version of an existing design, which attempts to tackle the three problems with the ZX81’s video:

• The ULA output is not designed for the 75 ohm load of a video input on a TV or monitor (so needs a transistor to buffer it).
• The voltage output of the ZX81 is too high compared to the normal video input on a TV or monitor.
• Two out of three ULA versions don’t generate a back porch.

The existing design is this one, see also this video. The PDF and the video date from July 2015.
This design incorporates the transistor buffer from earlier composite mods, plus adds the two diodes to reduce the voltage. Then added to this, is the 555 based monostable circuit to generate the back porch.

Do we know of any other home computers or games consoles that have the same three problems?

Quite why some of the component values were changed compared to the existing design, I don’t know. The change from a 820pF capacitor to a 1nF capacitor I can understand. 1nF are far easier to get hold of. But why replace a fixed 10 kΩ resistor with a preset for the base of the transistor on the output? That makes no sense, as it gives no useful adjustment. The transistor has enough gain to switch on regardless of the setting of the preset. And we want this transistor to switch off quickly. So we don’t ever want a lower base resistance.

Mark

They say there is always something to be learned and I have now just learned that the zeddies output is too strong, I always thought it was the other way around and the zeddie struggled so thanks for that gem which will now go in my knowledge locker,Mark very grateful .

It's troubling that the seller acted like mine must have been defective when in fact every single one he's sold will be like this. And it even does this with the good ULA. There must be dozens of them out there. Perhaps a lot of his customers buy them with the intention of fitting them at some point, and like Mark's one they sit unused for too long to complain about later.

One thing for sure, Patters is that if anyone can find a satisfactory solution it will be Mark rest assured.

My Was-to-be-thrown-in-the-skip-after-Moggy-ballsed-it-up ZX80 is still now chugging away thanks to his patient intervention.

So I now know what the major problem is. The NE555 supplied is inferior to a genuine NE555.

I’ve done a bunch of tests using a genuine TI NE555P (bought new from Rapid a couple of years ago), a genuine RCA CA555CE (from my own stock, bought from Maplin many, many years ago), a TA7555 from eBay (U.K. seller) and a eBay (China) 7555 (CMOS version). The module (with a couple of alterations) works with all these.

So I suspect that the supplied NE555P is a poor Chinese reproduction or maybe it was from a rejected batch.

I do recommend that for the best chance of getting a working unit, that a genuine CMOS 7555 version is used.

More later, including photos (I don’t have time now).

Mark

Fantastic - would this do the job ok? There aren't multiple specs of 555 are there (well apart from Chinese fakes)?
https://www.switchelectronics.co.uk/ne555p-timer-ic

I'll hang fire on an order until you confirm which other components I may need to order.

There are three types currently in production:

• NE555P standard bipolar type, the main manufacturer is Texas Instruments, but it is and has been manufactured by other companies under different part numbers. For example from ON Semiconductor under part number LM555CN.
• TLC555CP low power CMOS version from Texas Instruments.
• ICM7555IPA low power CMOS version from Intersil (now part of Renesas) or from Maxim under part no. ICM7555IPA+.

Although all should work, based on my testing so far, the ICM7555IPA has the greatest tolerance for circuit and ZX81 variations. It also has the greatest adjustment range, hence this is what I recommend.

I’ll give more details later.

Mark

This looks about right (using one of the small order suppliers recommended on the Stardot forum):
https://www.bitsbox.co.uk/index.php?mai ... ts_id=1284

patters wrote: ↑Fri Apr 02, 2021 1:23 am This looks about right (using one of the small order suppliers recommended on the Stardot forum):
https://www.bitsbox.co.uk/index.php?mai ... ts_id=1284

Yep, that's the type I recommend.

Mark